Numerical Investigation On Effects Of Combustion Chamber Geometry And Intake Conditions On Combustion And Emissions Of Marine Diesel Engine

With the energy crisis and environmental pollution becoming more and more serious,the marine diesel engine emission regulations are also more stringent,particularly in MARPOL convention on NOx emissions restriction.So under the condition of without damaging the engine power output,how to reduce nitrogen oxide emissions of diesel engine is a research hotspot.The combustion chamber geometry of diesel engine has significant impact on the in-cylinder flow and mixture formation,which is related to the efficient combustion and clean emissions of diesel engine,so the cylinder combustion optimization is the fundamental factor to determine the nitrogen oxide emissions.This paper carries out the optimization research of combustion chamber geometry and intake conditions by using simulation method.In this paper,with the help of CONVERGE software,a three-dimensional grid model was established based on a six-cylinder turbocharged inter-cooled marine diesel engine,which was used to study the effects of combustion chamber geometry,boost,EGR and intake valve closure timing(IVC T)on combustion,performance,and emissions of marine diesel engine.Five kinds of combustion chamber geometry were designed,including three kinds of reentrant geometry.The emphasis is to research the feasibility of reentrant geometry on medium-bored marine diesel engine.The results showed that double effects of reentrant geometry and fuel kinetic energy could strengthen the in-cylinder turbulent flow,promote the mixing of fuel and air,improve the proportion of premixed combustion and diffusive combustion rate and realize rapid burning.CA50 of reentrant geometry was smaller and its center of heat release rate was close to TDC.The smaller the ratio of diameter to depth and the reentrant ratio were,the higher the peak cylinder pressure,the indicated power and NOx emissions were,but the lesser soot emissions would be.With the increase of intake pressure,the peak cylinder pressure increased significantly,and the phase of peak cylinder pressure of reentrant geometry advanced,but the corresponding phase of prototype peak cylinder pressure was delayed.The NOx emissions of prototype showed the change tendency of increase after decrease,but that of soot emissions were just the opposite.The NOx and soot emissions of geometry(G5)with the minimum reentrant ratio were slightly higher than those of prototype under the intake pressure of 3.91 bar.Considering from the perspective of performance and emissions,the goal of high performance and low emissions could be achieved by using reentrant combustion chamber geometry with proper reentrant ratio and ratio of diameter to depth.As IVCT delayed and EGR rate increased,fuel ignition delay was prolonged and the maximum combustion pressure and NOx emissions went down.The strate gy of IVCT delaying 10 crank angle was better than that of 20% EGR ratio,but the reduction effect of IVCT delaying on NOx emissions was less remarkable than that of EGR ratio increase.As EGR rate increased,due to the effects of combustion chamber geometry on the ignition delay,eddy and inverse extr usion motion,the maximum instantaneous soot mass of prototype and G5 presented the opposite change tendency.After coupling boost,EGR and IVCT,G5 showed the capability to decrease NOx emissions and raise indicated power significantly on the premise of keeping maximum combustion pressure essentially constant.With respect to high performance and low NOx emissions,reentrant combustion geometry showed good applicability in medium-bored marine diesel engine.